(1) Field of the Invention
The present invention relates to security systems, and in particular to a security system for an electrical device connected to a mains electricity supply.
(2) Description of Related Art
According to the British Crime Survey 2004/2005, electrical goods and cameras were taken in 24% of burglaries, mobile phones were taken in 16% and computers and computer equipment were taken in 15% (data taken from http://www.homeoffice.gov.uk/rds/pdfs05/burglary—0405.xls). Electrical goods such as televisions, video equipment, stereo equipment and computers are all commonly left connected to a mains electricity supply in a home or office. Although items such as digital cameras, laptop computers and mobile phones are normally battery-powered, they are typically charged up periodically from the mains.
It is known to superimpose data signals on a mains electricity supply. Early applications of such technology were mostly limited to items such as intercoms and baby alarms. More recently, Power Line Communications (PLC) technology has been developed to enable both broadband Internet access over a mains supply and the creation of local area networks (LANs) within buildings. The HomePlug (RTM) Alliance has developed a set of standards for local area networking across domestic power lines and has certified products from a wide variety of manufacturers. Chipsets for HomePlug devices are now readily and cheaply available, which makes networking in a home or office via a mains power supply affordable.
Current intruder alarms, which typically use sensors such as door switches and passive infrared (PIR) devices, are of limited use in environments normally accessible to the public, such as offices, shops, hotels, schools, colleges, museums and libraries. The physical locking devices sometimes used to secure electrical devices in these environments can be considered clumsy and unsightly. Furthermore, conventional movement sensors such as PIR devices can be either under-sensitive to the presence of an intruder, or over-sensitive to the movement of, for example, pets.